Optogenetics is a technical methodology that allows direct light-based manipulation of genetically specified cells. Optogenetic methods have provided novel insights into the role of defined neuronal populations in brain function and animal behavior. An expanding palette of single-component optogenetic tools provides powerful interventional strategies for modulating the function of targeted neurons in awake, behaving mammals and for detailed interrogation of circuit physiology in vitro. Although several genetic methods can be utilized for delivering these genes into target cell populations, the use of viral vectors for delivery of optogenetic tools has several important advantages. In recent years, techniques for viral vector-mediated delivery of optogenetic tools have improved and expanded significantly. These techniques now allow modular use of optogenetic tools in defined cell types and circuits and dovetail well with genetic mouse models and recombinase-based driver lines. Here, we review the use of viral vectors for delivering genes encoding optogenetic tools into the rodent brain and provide a detailed protocol for viral transduction of mouse cortical neurons and chronic implantation of a fiberoptic connector for light delivery in vivo .